Imaging apparatus, such as photographic film cameras and electronic cameras, and in particular their optical assemblies, have inherent aberrations which can degrade the quality of images captured by such apparatus. One kind of aberration is a distortion, which refers to a change in the geometric representation of an object in the image plane. For instance, a rectangle might be reproduced with a pincushion or a barrel shape—hence the reference to pincushion distortion or barrel distortion. Another type of aberration, referred to as chromatic aberration, results from the fact that different wavelengths or colors of light are refracted by different amounts by an optical assembly. A further type of aberration is a field dependent aberration, where some characteristic, such as the brightness, of an image pixel is changed in the image plane in proportion to its position in the field, such as its distance from the center of the image.
Chromatic aberration appears when a lens is transmitting polychromatic light (many colors). Since the index of refraction of optical glass is wavelength dependent, the red, green and blue components bend differently at an optical interface in the lens. This leads to longitudinal (axial) and/or lateral chromatic aberration effects. When a lens fails to focus various colors sharply in the same plane, the lens is said to exhibit longitudinal (axial) chromatic aberration. In longitudinal chromatic aberration, the three components are brought to focus on different planes in the image space, which gives a color blurring effect. Thus, longitudinal chromatic aberration arises due to the focal length varying with wavelength (color). In lateral chromatic aberration, color components from a single point are brought to focus to different points on the same image plane, resulting in a lateral shift of the image. This has the effect of magnifying the three colors differently and can be visually seen as color fringing. Thus lateral chromatic aberration can be seen as an effect due to magnification varying with wavelength.
A great deal of the complexity of modern lenses is due to efforts on the part of optical designers to reduce optical aberrations. In certain cases, such as with single use film cameras or inexpensive digital cameras, it may be economically difficult to avoid usage of inexpensive optics. Unfortunately, as explained above, such optics possess inherent aberrations that degrade the quality of images formed by the optics. Consequently, it is desirable to compensate for these aberrations in the reproduction process (either in the capture device or in a host computer) so that final images free of aberrations may be obtained. In order to characterize these aberrations, the ability of a lens to transfer information from the object to an image plane is represented as a modulation transfer function (MTF). A lens MTF is a measure of how well the original frequency-dependent contrast of the object is transferred to the image.
In a typical camera, in addition to distortion and chromatic aberrations, the image formed at a focal plane (where the film or image sensor is located) can be blurred as a function of proximity to the optical axis of the optical assembly. For such field dependent aberrations, the further away from the optical axis (normally, the center of the image), the more the image is blurred. The resultant image therefore has an MTF that is a function of radial distance from the center of the image. The problem is exaggerated with images originating from inexpensive cameras, such as single use film cameras. Because of their simple optics or because the film may not be located in the position of best focus throughout the focal plane, single use film cameras tend to have significant sharpness loss with movement away from the optical axis toward the edges of the frame. Consequently, it is also desirable to compensate for these aberrations in the reproduction process (either in the capture device or in a host computer) so that final images free of field dependent aberrations may be obtained.
In one example, a camera system described in U.S. Pat. No. 5,461,440, entitled “Photographing Image Correction System” and issued Oct. 24, 1995 in the names of Toyoda et al., does not require an expensive optical assembly that is corrected for marginal attenuation (light amount irregularity) and distortion (pincushion and barrel distortion). Instead, the curvature of field data and the light amount irregularity data corresponding to the optical assembly are identified in advance, and stored either in the camera or separately at a downstream scanning and processing station. Either way, the data is linked to the specific camera and then used in subsequent film processing and scanning to correct the image signal for the image quality degradation imparted by the optical assembly.
The image quality of captured images can be improved by the selection of appropriate filters for the input imaging device and subsequent devices that process the captured images. For instance, in U.S. Pat. No. 4,970,593, entitled “Video Image Enhancement Utilizing a Two-dimensional Digital Aperture Correction Filter” and issued Nov. 13, 1990 in the name of C. Cantrell, the modulation transfer function (MTF) of the uncorrected optical system is measured and an aperture correction function is created from an inverse of the MTF function to correct an image captured through the optical system. In commonly-assigned U.S. Pat. No. 5,696,850, entitled “Automatic Image Sharpening in an Electronic Imaging System” and issued Dec. 9, 1997 in the names of Kenneth Parulski and Michael Axman, a digital image produced by a digital camera is improved by using a sharpening filter that is produced as a function of the system MTF. Although these arrangements produce an improved image, there are still problems with image quality. For example, the image can still suffer from position dependent blur and channel dependent blur.
Commonly assigned U.S. Pat. No. 6,628,329, entitled “Correction of Position Dependent Blur in a Digital Image” and issued Sep. 30, 2003 in the names of Sean C. Kelly, Donald Williams and David Jasinski, describes the correction of position dependent blur in a digital camera, where the position dependence is a function of the proximity of a pixel to the optical axis. Typically, the camera manufacturer measures the MTF at various locations in the image, and then creates a boost map that is applied to a sharpening kernal to adjust for position blur of the captured image. The boost value at each of the pixels of the image is inversely proportional to the actual MTF, i.e., equal to a desired MTF value divided by the actual MTF value for that pixel. It is desirable that this technique be used to spatially equalize the sharpness, to correct for lens sharpness roll off. This technique is also useful in purposefully modifying the local MTF to some different aim (either inducing local blur or enhanced sharpness).
Some aberrations, specifically chromatic aberrations, are channel dependent aberrations in the sense that each color channel, e.g., red, green and blue channels, provides a different amount of the aberration artifact in the image plane. It has also been observed that some field dependent aberrations, such as position dependent blur, are also channel dependent. Consequently, a different amount of correction would ideally be provided for each color channel at the image plane. For instance, lens designers typically provide complicated, and therefore expensive, designs to differentially control the light rays according to wavelength in order to minimize such artifacts.
Especially if they are intended for consumer use, digital cameras, which are inherently more complex and expensive than simple film cameras, such as single use film cameras, must control cost in any way possible. The camera optics is a typical candidate for cost reduction, and channel-dependent artifacts thus becomes a concern. Despite such image quality concerns, it is usually desirable to provide a finished image file that is corrected for camera-related influences. What is needed is a simple correction for channel dependent aberrations, such as channel dependent blur and sharpness fall-off, that does not require a more complex, or more expensive, optical system, as well as a correction that can be implemented in the processor of a digital camera, or in the downstream scanning and processing of a film system. More specifically, a simple correction is needed for the kind of channel dependent blurring caused by longitudinal chromatic aberration and field dependent effects.
Channel dependent corrections for a printing process are addressed in commonly assigned U.S. Pat. No. 6,728,003, entitled “Method of Compensating for MTF in a Digital Image Channel,” and issued Apr. 27, 2004 in the names of Andrew Gallagher and Robert Parada. In this patent, a digital image comprises a plurality of digital image channels, such as red, green and blue channels. A degradation in the MTF of a device in an imaging chain is compensated by using the MTF and a gain factor to provide an aim response, generating a filter from the aim response, and then using the filter to process the image channel. In published U.S. Patent Application 2004/0218071, entitled “Method and System for Correcting the Chromatic Aberrations of a Color Image Produced by Means of an Optical System” and published Nov. 4, 2004 in the names of Chauville et al., a system and method is described for correcting the chromatic aberrations of a digital image composed of a plurality of color planes. The geometric anomalies, especially distortions, of the digitized color planes are modeled and corrected, at least partly, in such a way as to obtain corrected color planes. The corrected color planes are then combined in such a manner as to obtain a color image corrected completely or partly for the distortion-based chromatic aberrations. Neither Gallagher et al. nor Chauville et al. address longitudinal chromatic aberrations or field dependent artifacts.
What is therefore needed is a method for removing the aberration of longitudinal color in captured digital images. In particular, there is need for a restoration algorithm that can remove the negative imaging artifacts associated with the aberration of longitudinal color. These negative artifacts include color fringing wherein the three color planes do not line up on top of each other, and sub-optimal sharpness in at least one of the color channels.